This invention relates to novel cationic starch-graft-copolymers having copolymeric side chains comprising copolymers of N,N'-methylenebisacrylamide and various polyamines and to methods for the preparation of such starch-graft-copolymers. This invention also relates to starch-graft-copolymers which contain, in addition to the cationic copolymeric side chain, other substituent groups on the starch base. This invention further relates to the use of these novel cationic starch-graft-copolymers in various papermaking processes, especially to their use as pigment retention aids.
As used herein, the term "starch-g-poly(N,N'-methylenebisacrylamide-co-amine) copolymer" refers to a starch base having copolymeric side chains grafted onto the hydroxyl groups of the starch molecule, wherein the grafted side chains, straight or branched, are copolymers of N,N'-methylenebisacrylamide and various polyamines and the copolymeric side chains are added to the starch base through the opening of the double bond of N,N'-methylenebisacrylamide. The term "polyamine" refers to amines containing at least two amine groups selected from the group consisting of primary and secondary amines. In addition, the polyamines may contain unreactive tertiary nitrogen atoms and may be straight chain, branched, or cyclic polyamines.
As used herein, the term "paper" refers to sheet-like masses and molded products made from fibrous cellulosic materials which may be derived from natural sources, as well as from synthetics such as polyamides, polyesters, and polyacrylic resins, and from mineral fibers such as asbestos and glass. In addition, papers made from combinations of cellulosic and synthetic materials are applicable herein. Paperboard is also included within the broad term "paper".
It is well known to form graft copolymers of starch by free-radical-initiated polymerizations, in some cases by anionic polymerizations, and in a few cases by grafting preformed homopolymers onto starch derivatives or starch. A discussion of such polymerization methods may be found in "Block and Graft Copolymers", ed. by R. J. Ceresa (New York: John Wiley & Sons, 1973), pp. 1-24.
It is well known to use starch and starch derivatives as aids in various paper making processes such as wet end, sizing, and coating applications. It is also well known to use cationic starch ether derivatives as pigment retention aids. U.S. Pat. No. 3,962,159 suggests the use, as wet strength additives for paper, of starch graft copolymers formed by a condensation reaction of starch and preformed polyamide-amines.
It is an object of the present invention to provide novel cationic starch-graft-copolymers, useful as pigment retention aids, and to provide an "in situ" and "preformed copolymer" method for their preparation from N,N'-methylenebisacrylamide and various polyamines by a non-radical addition reaction.